Method for monitoring adjacent zones in a mobile radio telephone system and a corresponding mobile radio telephone system

ABSTRACT

A method and system are provided for monitoring adjacent cells in a mobile radio telephone system, in which a mobile station located in a cell of a mobile radio telephone system receives, for purposes of monitoring adjacent cells, transmits signals coded with different cell-specific codes from base stations of adjacent cells and codes and evaluates these in accordance with the respective cell-specific codes, wherein at least the base station currently responsible for the mobile station transmits a signaling information item which informs the mobile station of the codes to be used in the monitoring of adjacent cells and/or of the codes not to be used in monitoring adjacent cells, and the mobile station, in monitoring adjacent cells, only decodes and evaluates the transmit signals of the adjacent cells coded with a code which can be used for the monitoring of adjacent cells in accordance with the signaling information.

BACKGROUND OF THE INVENTION

It is known that conventional mobile radio telephone systems are ofcellular configuration; i.e., the respective mobile radio telephonesystem is formed by a multiplicity of radio cells and a base station isallocated to each radio cell. If a mobile station (for example, a mobiletelephone), is located in the geographic area of a radio cell,communication with the mobile station within the mobile radio telephonesystem takes place between the mobile station and the base stationallocated to the current cell. If the mobile station is moving from onecell into another cell, the communication link existing between themobile station and the base station allocated to the “old” cell must behanded over to the base station allocated to the “new” cell. Thisprocess is called “handover”.

So that such a handover can be performed as rapidly and as free ofinterference as possible, it is necessary for the mobile station locatedin the geographic area of a cell to continuously monitor and evaluatethe signals transmitted by the base stations of adjacent cells in orderto obtain system information of the adjacent cells which can be used forsetting up communication between the base station of the new cell andthe mobile station as rapidly as possible when entering into an adjacentnew cell.

This monitoring of adjacent cells by the mobile station is made moredifficult due to the fact that the base stations in the individual cellsuse different scrambling codes. For the mobile station located in acell, this means that it must be continuously adjusted for decoding andevaluating the transmit signals from base stations of adjacent cells inaccordance with the different-scrambling codes. The more adjacent cellsthere are, the longer the monitoring of adjacent cells will take.

At national boundaries which, as a rule, also represent separating linesbetween two different mobile radio telephone network operators, the casemay occur that the two network operators of different nationalities usethe same frequency band. If a mobile station of one network operator isclose enough to the border of the other network operator, the mobilestation can receive the transmit signal of the cell of the other networkoperator on the other side of the border. In UMTS (Universal MobileTelecommunication System) systems, for example, the signals received bythe mobile station are unambiguously allocated to a cell or to a networkoperator by reading a system information item which is transmitted viathe so-called broadcast control channel (BCCH) of the base station ofthe respective cell.

Reading the system information item of an adjacent cell is atime-consuming process so that, for example, at a national boundary, thecase may occur that there is only one adjacent cell of the same operatorfor a cell in which the mobile station is currently located whereasthere are several adjacent cells of the other network operator so thatthe system information items of the adjoining cells of the other networkoperator must be read and evaluated during the monitoring of adjacentcells. In other words, the mobile station needs a large amount of timefor identifying the signal received from a network operator.Cumulatively, this time can become very large if there is a very largenumber of adjacent cells of a foreign network operator transmitting atthe same frequency as one's own network operator; i.e., if a very largenumber of adjacent cells of a foreign network operator are visible atthe same frequency for the mobile station.

To illustrate, a dashed national boundary is shown in FIG. 5 whichseparates a cellular mobile radio telephone network of a networkoperator A and a cellular mobile radio telephone network of a networkoperator B. According to FIG. 5, the mobile radio telephone network Aincludes individual cells A0–A6 whereas the mobile radio telephonenetwork B includes individual cells B0–B6. If, for example, a mobilestation of network operator A is located in cell A6, the mobile stationcan monitor both the adjacent cells A3 and A4 of its own mobile radiotelephone network A and the adjacent cells B0, B3, B4 and B1 of theforeign mobile radio telephone network B. That is, the mobile stationreads both system information of cells A3 and A4 and system informationof cells B0, B3, B4 and B1 even though this would actually only berequired for cells A3 and A4 since cells B0, B3, B4 and B1 are cells ofa foreign mobile radio telephone network.

The present invention is directed toward providing a method formonitoring adjacent cells in a mobile radio telephone system, and acorrespondingly designed mobile radio telephone system, via which thetime needed for monitoring adjacent cells can be reduced.

SUMMARY OF THE INVENTION

According to the present invention, signaling information is transmittedto the mobile station located in a cell from the base station of thecorresponding cell, which signaling information informs the mobilestation about the scrambling codes to be used or not to be used duringthe monitoring of adjacent cells. In this manner, the transmit signalsof the adjacent cells, to be observed during the monitoring of adjacentcells, or, respectively, the corresponding scrambling codes can bedelimited and thus the monitoring of adjacent cells can be acceleratedin that the mobile station is only informed about the scrambling codesused by the adjacent cells of its own network operator and,respectively, the scrambling codes used by the adjacent cells of aforeign network operator are excluded from the monitoring of theadjacent cells. This assumes that the individual adjacent cells in eachcase use different scrambling codes which is necessary in the case ofadjacent network operators which use the same frequency, even inaccordance with the current state of the art, by using common advancedplanning of the scrambling codes used in the individual cells.

The present invention is not restricted to the preferred field ofapplication of the scrambling codes but can be applied generally to allpossible types of cell-specific codes via which the base stations of theindividual cells generate a transmit signal containing, in particular,the system information of the corresponding cell, the present inventionbeing applicable, in particular, in UMTS mobile radio telephonenetworks.

The signaling information informing the mobile station of the codes tobe used or not to be used in the monitoring of adjacent cells ispreferably provided in the system information of the current cell. Inthis case, in principle, it would be sufficient if this signaling istransmitted in each case in the cells of a mobile radio telephonenetwork close to the border; i.e., in the cells immediately adjoininganother mobile radio telephone network.

So that it is not necessary to report all codes to be used or not to beused for the monitoring of adjacent cells with the aforementionedsignaling information, this information also can be transmitted incorrespondingly coded form in order to reduce the signaling expenditure.

Thus, for example, it is possible to use the signaling information toreport only the boundaries of the area in which the scrambling codes tobe used or not to be used for the monitoring of adjacent cells arelocated. In general, all scrambling codes used in the adjacent cells canbe combined in subsets and only an information item designating therequired subset can be transmitted so that, in principle, only a numberfor designating the required subspace or the required subset needs to betransmitted in this exemplary embodiment.

Another exemplary embodiment which allows a relatively large amount offreedom to the network operators when issuing the scrambling codes andwhich can be coded using relatively few bits provides mapping ofindividual decimal scrambling codes of the adjacent cells in binary formand selection of certain bits of the binary scrambling codes with theaid of a corresponding mask. If the bits of the individual scramblingcodes selected by the mask correspond to a corresponding pattern, thismeans for the mobile station that the corresponding scrambling codes areto be used or not to be used in monitoring the adjacent cells. In thistype of signaling, only the mask and the required pattern need to bereported to the mobile station in the form of system information.

According to a further exemplary embodiment of the present invention, itis proposed to use scrambling codes which are in each case aligned withthe same compass directions in the individual mobile radio telephonenetworks or countries. In particular, this relates to the scramblingcodes used in the cells close to the border. In the interior of thecountry, in contrast, arbitrary scrambling codes can be used. Thealignment of the scrambling codes oriented in accordance with thecompass directions can be effected, for example, in such a manner thatcertain bits of the scrambling codes to be used or not to be used formonitoring the adjacent cells are specified in each case at the cornersof the individual mobile radio telephone networks corresponding to thesame compass direction. Thus, for example, the last two bits of thescrambling codes to be used or not to be used for monitoring theadjacent cells can be specified in each case, for example, at thecorners of the individual mobile radio telephone networks whereas onlyone of these last two bits of the scrambling codes is specified in thecells close to the border between two corners of a mobile radiotelephone network; i.e., in a transition zone between two corners. Inthis manner, only the current geographic position of the mobile stationwithin the respective mobile radio telephone network needs to betransmitted for signaling the scrambling codes to be used or not to beused in monitoring the adjacent cells which only requires three bits.Depending on this information about its own current geographic position,the mobile station can draw direct conclusions about the scramblingcodes to be used in each case for monitoring the adjacent cells.

Using this exemplary embodiment, it is possible to reduce the timerequired for monitoring adjacent cells with particularly littlesignaling expenditure and, moreover, collisions can be reliably avoidedsince due to the identical alignment of the scrambling codes inaccordance with the compass directions, provided in the individualadjacent mobile radio telephone networks or countries, the samescrambling codes cannot be used in adjacent cells or different networks.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the following DetailedDescription of the Invention and the Figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a diagrammatic representation for explaining, via anexemplary embodiment, the principle forming the basis of the presentinvention.

FIG. 2 shows a chart for explaining a further exemplary embodiment ofthe present invention.

FIGS. 3 and 4 show representations for explaining a further exemplaryembodiment of the present invention.

FIG. 5 shows a cell mobile radio network for explaining the problemforming the basis of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, two mobile radio telephone networks of a network operator Aand of a network operator B, separated by a national border, are shown.Mobile radio telephone network A includes radio cells A0–A3 and mobileradio telephone network B includes radio cells B0–B2. To each cell, abase station is allocated which is responsible for communication with amobile station within the geographic area of the corresponding cell.FIG. 1 only shows the corresponding base station BS for cell A0. It isassumed that a mobile station MS is currently located in cell A0.

In each cell, the corresponding base station uses another scramblingcode, assuming that the use of scrambling codes of the two adjoiningnetwork operators A and B has been jointly planned. FIG. 1 shows thescrambling codes used by the corresponding cells in the form SC(n),where SC(n) is the scrambling code with the ordinal number n.

Each base station of the individual cells, particularly the basestations of cells A0, A1 and A3 and, respectively, B0–B2, which areclose to the border, have a list of the scrambling codes used in theadjacent cells of the same network operator and the scrambling codesused in adjacent cells of the adjoining network operator. In the exampleshown in FIG. 1, this means for cell A0 or the corresponding basestation BS that in this list, scrambling codes n=(12, 62, 112) aremarked as scrambling codes of its own network operator and scramblingcodes n=(39, 43, 57) are marked as scrambling codes of a foreign networkoperator. Transmitting this list in the form of corresponding signalinginformation to the mobile station MS located in the geographic area ofcell A0 informs the mobile station MS that only the scrambling codeswith n=(12, 62, 112) are to be used for monitoring adjacent cells andscrambling codes with n=(39, 43, 57) are not to be used for monitoringadjacent cells.

Depending on the network topology, however, this list with own orforeign scrambling codes can become relatively long so that thesignaling complexity also correspondingly can become relativelyextensive. In the text which follows, therefore, exemplary embodimentsare shown in which signaling of the scrambling codes to be used or notto be used for monitoring adjacent cells is possible with reducedexpenditure.

Thus, for example, it is possible to use the signaling informationtransmitted from the base station BS of cell A0 to mobile station MS toreport only an area in which the scrambling codes to be used or not tobe used for monitoring adjacent cells are located. Thus, it is onlynecessary to transmit two numbers, namely the beginning and the end ofthe corresponding area, to the mobile station MS. For the example shownin FIG. 1, this means that the base station BS only needs to transmitthe area boundaries “39” and “57” to the mobile station MS in order toreport the scrambling codes to be used for monitoring adjacent cells.

A further possibility for signaling with little expenditure issubdividing the scrambling codes used in the adjacent cells into anumber of subspaces or subsets so that it is only necessary to transmita number for designating the subset with the scrambling codes requiredfor monitoring adjacent cells to the mobile station MS. In the exampleshown in FIG. 1, for example, the scrambling codes of the cells A1–A3 ofthe same mobile radio telephone network A, which are adjacent to cellA0, and the scrambling codes of the cells B0–B2, which are adjacent tocell A0, of the foreign mobile radio telephone network can be combinedin subsets in such a manner that the scrambling codes of the adjacentcells of their own mobile radio telephone network A all have theproperty n=even where the scrambling codes of the adjacent cells of theforeign mobile radio telephone network B have the property n=odd.

At points at which more than two different mobile radio telephonenetworks meet one another, this dividing of the scrambling codes can becontinued where, for example, the scrambling codes of the adjacent cellsof their own mobile radio telephone network A generally meet theproperty n mod p, p being the number of meeting mobile radio telephonenetworks.

FIG. 2 shows a further exemplary embodiment of the present invention forsignaling the scrambling codes to be used in monitoring adjacent cellswith little expenditure, the example shown in FIG. 2 allowing arelatively large amount of freedom to the network operators in issuingthe scrambling codes and being codeable with a relatively small numberof bits.

FIG. 2 shows the individual scrambling codes shown in FIG. 1 and thecorresponding ordinal numbers of the cells adjacent to cell A0 both indecimal and in binary form. A mask m is defined which only selectscertain bits of the individual binary scrambling codes. If the bitsselected by the mask m form a particular pattern, the pattern “011” inthe exemplary embodiment shown in FIG. 2, this means for the mobilestation MS that the corresponding scrambling code is not to be used inmonitoring adjacent cells.

In the exemplary embodiment shown in FIG. 2, with m of length 8 bits, itis only necessary to specify the mask m with 8 bits and the pattern with3 bits to be monitored for coding the system information. Theinformation of the mask m or, respectively, of the corresponding rule,which 3 bits are to be selected from the 8 bits of the ordinal number nof the individual scrambling codes, could be coded with only 6 bitssince there are only 56 possibilities of selecting three bits from eightbits.

FIG. 3 and FIG. 4 show a further exemplary embodiment of the presentinvention which enables the time required for monitoring adjacent cellsto be shortened with particularly low signaling expenditure.

The assumption is here that the scrambling codes used in the individualcells are defined in accordance with the arrangement shown in FIG. 3 inall mobile radio telephone networks or countries; particularly, in themobile radio telephone networks or countries adjoining one another. FIG.3 always indicates, by way of example, the allocation of the last twobits of the binary scrambling codes or, respectively, a numberassociated with this allocation, the individual rectangles in each caseincluding groups of cells.

As is shown in FIG. 3, various scrambling codes are permitted within acountry or mobile radio telephone network since the last two bits ofthese scrambling codes are occupied by “xx”. Whereas the assignment isarbitrary in the interior of the country, the last two bits of thescrambling codes used in the corresponding cells are specified in thefour corners of the country or mobile radio telephone network,respectively. Thus, for example, only scrambling codes the last two bitsof which are occupied by “00” are used in all cells corresponding to thenorth-eastern corner, etc. For the edge or border areas, in contrast,only one of these bits is specified so that, for example, onlyscrambling codes, the last-but-one bit of which is occupied by “0” areused for the cells which are located close to the border between thenorth-eastern corner and the south-eastern corner.

If the mobile radio telephone network or country considered in each casedoes not have a rectangular shape according to FIG. 3, the principlemust be correspondingly generalized.

Naturally, a different association between the directions and possiblescrambling codes is also possible; e.g., the arrangement according toFIG. 3 can be rotated and/or mirrored or it can be generalized to alarger number of directions, (e.g., 8 directions specified by 3 bits ofthe scrambling code).

To inform the mobile station, which, for example, is located in a cellclose to the border, of the scrambling codes to be used for monitoringadjacent cells, it is only necessary to inform the mobile station of anidentifier designating the current geographic position of the mobilestation within the corresponding mobile radio telephone network. Since,according to the arrangement shown in FIG. 3, a distinction is madebetween eight different compass directions, 3-bit signaling is adequatefor this. If, for example, the mobile station is informed in this waythat it is located in the north-eastern corner of the correspondingmobile radio telephone network, the mobile station can derive from thisfact that it should only use scrambling codes the last two bits of whichare occupied by “00” for monitoring adjacent cells. All other scramblingcodes are not used by the mobile station for monitoring adjacent cells.

FIG. 5 shows for the example of European mobile radio telephone networksor, respectively, European mobile radio telephone network operators howthe arrangement shown in FIG. 4 can be mapped onto the individualcountries or mobile radio telephone networks. In each case, thenorth-eastern corner is designated by “0”, the south-eastern corner by“1”, the north-western corner by “3” and the south-western corner by “2”(compare also FIG. 3).

As also can be seen from FIG. 4, it is not possible for collisions tooccur; i.e., it is possible for the same scrambling codes to be used inadjacent cells of different countries or mobile radio telephone networkoperators since the same arrangement (compare FIG. 3) is used in allcountries or mobile radio telephone networks for allocating orspecifying the scrambling codes, and a national border or corner atwhich a number of countries meet cannot be located in the same compassdirection seen from all the countries. For example, a point cannot beinterpreted at the same time as the south-eastern corner by two (ormore) adjacent countries or a border cannot be simultaneously thesouthern border of two countries.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the present invention as set forth in the hereafter appended claims.

1. A method for monitoring adjacent cells in a mobile radio telephonesystem, in which a mobile station located in a cell of the mobile radiotelephone system receives, for purposes of monitoring adjacent cells,transmit signals coded with different cell-specific codes from basestations of adjacent cells and codes and evaluates the transmit signalsin accordance with the respective cell-specific codes, the methodcomprising the steps of: transmitting, via at least the base stationcurrently responsible for the mobile station, signaling informationwhich informs the mobile station of at least one of the codes to be usedin the monitoring of adjacent cells and the codes not to be used inmonitoring adjacent cells, wherein the signaling information informs themobile station of a particular mask and a particular bit pattern; anddecoding and evaluating, via the mobile station in monitoring adjacentcells using the mask and/or bit pattern, the transmit signals of theadjacent cells coded with a code which can be used for the monitoring ofadjacent cells in accordance with the signaling information.
 2. A methodfor monitoring adjacent cells in a mobile radio telephone system asclaimed in claim 1, wherein the codes are scrambling codes.
 3. A methodfor monitoring adjacent cells in a mobile radio telephone system asclaimed in claim 1, the method further comprising the step oftransmitting the signaling information via at least base stations ofcells arranged at a border between one mobile radio telephone networktelephone operator and another mobile radio telephone network operator,in which one of the codes to be used in accordance with the signalinginformation from monitoring adjacent cells correspond to adjacent cellsof the same network operator, and the codes not to be used according tothe signaling information for monitoring adjacent cells correspond toadjacent cells of the other network operator.
 4. A method for monitoringadjacent cells in a mobile radio telephone system as claimed in claim 1,wherein the signaling information is transmitted as system information.5. A method for monitoring adjacent cells in a mobile radio telephonesystem as claimed in claim 1, wherein each base station transmitting thesignaling information has at least one of a list about the codes to beused in monitoring the adjacent cells and a list about the codes not tobe used in monitoring the adjacent cells.
 6. A method for monitoringadjacent cells in a mobile radio telephone system as claimed in claim 1,wherein the signaling information informs the mobile station of an areain which one of the codes to be used and the codes not to be used formonitoring the adjacent cells are located.
 7. A method for monitoringadjacent cells in a mobile radio telephone system as claimed in claim 1,the method further comprising the steps of: combining the codes used inthe adjacent cells in subsets, with one subset including the codes to beused in monitoring the adjacent cells; and informing the mobile station,via the signaling information, of the subset with the code to be used inmonitoring the adjacent cells.
 8. A method for monitoring adjacent cellsin a mobile radio telephone system as claimed in claim 1, whereincertain bits are selected according to the mask from a binaryrepresentation of the individual codes of the adjacent cells, and themobile station uses for the monitoring of adjacent cells only codes thebits of which, selected by the mask, one of correspond to the particularbit pattern and do not correspond to the particular bit pattern.
 9. Amethod for monitoring adjacent cells in a mobile radio telephone systemas claimed in claim 1, the method further comprising the steps of:defining, in the individual cells of the mobile radio telephone system,the codes used by the respective base station in accordance with ageographic position of the respective cell in the mobile radio telephonesystem; informing the mobile station, via the signaling information, ofa current geographic position of the mobile station within the mobileradio telephone system; and drawing conclusions, via the mobile station,about the codes used in the adjacent cells about its own geographicposition within the mobile radio telephone system and using the codesfor monitoring the adjacent cells.
 10. A method for monitoring adjacentcells in a mobile radio telephone system as claimed in claim 9, whereinfor cells arranged within a same geographic area of the mobile radiotelephone system, at least one bit of the code used in the correspondingcells is identically specified, two bits being identically specifiedparticularly for cells located in corner areas of the mobile radiotelephone system whereas at least one bit of the codes used in the cellsis identically specified for cells of the mobile radio telephone systemarranged in edge areas with one another.
 11. A method for monitoringadjacent cells in a mobile radio telephone system as claimed in claim 9,wherein a plurality of adjacent mobile radio telephone systems areprovided and, in each mobile radio telephone system, the codes used inthe corresponding cells are defined in accordance with a samearrangement in accordance with the geographic position of the respectivecell in the respective mobile radio telephone system.
 12. A mobile radiotelephone system, comprising: a plurality of cells; a plurality of basestations, each cell being associated with one of the base stations, witheach base station generating a transmit signal coded with a particularcell-specific code; and a mobile station located in a particular cell ofthe mobile radio telephone system, for monitoring adjacent cells,receiving the transmit signals from the base stations of adjacent cells,and decoding and evaluating the transmit signals in accordance with therespective cell-specific code; wherein at least the base stationcurrently responsible for the mobile station, of the cell in which themobile station is currently located, transmits signaling informationwhich informs the mobile station of at least one of the codes to be usedin monitoring the adjacent cells and the codes not to be used inmonitoring the adjacent cells and wherein the signaling informationinforms the mobile station of a particular mask and a particular bitpattern, and wherein the mobile station receives and evaluates thesignaling information and, in monitoring the adjacent cells, onlydecodes and evaluates the coded transmit signals of the adjacent cellsaccording to the mask and/or bit pattern which can be used formonitoring the adjacent cells in accordance with the signalinginformation.
 13. A mobile radio telephone system as claimed in claim 12,wherein the mobile radio telephone system is a UMTS mobile radiotelephone system.